most people purchase springs because it is cheaper than REAL coilover kits. yes a real coilover kit includes the spring,perch, sleave, and shocks/struts. However most coilover kits you see are coilover sleeves and only include the spring/perch/sleeve.
i've heard that the groundcontrol bottoms out and cracks the top of the sleeve. if you slam your ride(which is usually the case w/ coilovers) then you will most defintely need a camber kit to keep your tires from balding unevenly and too fast.
coilovers allow adjustment whenever you want (which is the good part). if you decide on purchasing the groundcontrol coilover sleeve kit, get some aftermarket shocks also because you're gonna blow out your stock shocks.